1. Computer Science is not taught in US schools
Schools that teach computer science courses are very limited. I, for example, did not know anything about the subject until my first semester in college.
2. As an elective, it doesn't contribute to graduation requirements
This means less students are willing to take it as an elective, limiting their exposure to the subject.
3. The nerd stereotype is proven to drive away women
Technology and Computer Science/Engineering has a stereotype of being an area for "nerds". If girls do not relate to this subgroup, then they feel that they will not fit in or be accepted.